A heat radiating tube as a component of a heat radiating tube burner for an annealing furnace is exposed to a high temperature combustion gas and, therefore, the thickness of the radiating tube is markedly reduced due to the occurrence of high temperature oxidation corrosion on the inside of the heat radiating tube. As a result the life expectancy of the heat radiating tube is shortened. Heretofore, in an effort to inhibit such high temperature oxidation corrosion, the heat radiating tube conventionally is made of a Fe-Cr-Ni heat resistant cast alloy, including over 20% of Cr and 10-35% of Ni, so that a Cr.sub.2 O.sub.3 protecting oxide-film is formed on the surface thereof. (Reference: J. J. Jones, "Development in Heat-resisting Alloys for Petrochemical Plant", Research and Development of High Temperature Materials for Industry, Elsevier Applied Science, p.31, 1989).
However, the Cr.sub.2 O.sub.3 oxide products are vaporized in the form of CrO.sub.3 at temperatures over 900.degree. C., and therefore, there is a limit in using Cr.sub.2 O.sub.3 as the material for the protecting oxide film of the heat radiating tube which is exposed to a high temperature combustion gas.
In order to solve the above described problem of the Cr.sub.2 O.sub.3 protecting film, there has been developed a FeCrAl alloy which forms a stable Al.sub.2 O.sub.3 even at temperatures over 900.degree. C. In addition, an FeCrAlX alloy has been developed in which the adhesive strength of the Al.sub.2 O.sub.3 is improved. (Related patent: SE874859).
Further, in order to achieve a sustained formation of the Al.sub.2 O.sub.3 protecting film on the surface, an aluminum diffusion coating method is used. (related patent: U.S. Pat. No. 3,762,884).
However, in the case where the radiating tube is constituted in a W-shaped form as shown in FIG. 1, the combustion gas which is burned in a burner 1 of FIG. 1 flows in parallel with the inside surface of the tube during the passage through a straight tube portion 2. However, during the passage through a curved portion 3, the gas flow rates for the radially outer portion of the curved tube portion and the radially inner portion of the curved tube portion differ from each other. Accordingly, due to the difference between the gas flow rates, the combustion gas cannot flow in parallel with the inside surface of a second straight tube portion 4 at the initial portion thereof. As a result, a combustion gas flame collision phenomenon occurs at the upper initial portion 5 of the second straight portion of the W-shaped heat radiating tube.
Further, due to the instability of the combination between the fuel gas and air in the burner, the aforementioned combustion gas flame collision phenomenon can occur also in the first straight tube portion 1. Thus the portions where the flame collisions occur are subjected to a high temperature, and undergo an extremely severe thermal stress. Consequently, the Cr.sub.2 O.sub.3 or Al.sub.2 O.sub.3 protecting films which are formed on the inside of the tube gradually are destroyed, which then causes a rapid reduction in the tube thickness due to the oxidation corrosion. Ultimately, that the flame collision portions produce holes. Thus, the prior heat resistant alloys using a protective oxide films, as well as the metal diffusion coating method for improving the formation efficiency of the protecting films, have a limit in preventing high temperature oxidation corrosion caused by the combustion gas flame collision phenomenon.